1. Field of the Invention
The present invention relates to a silicon nitride sintered body excellent in heat resistance under high temperature environments and a method of producing the same. More particularly, the present invention relates to a silicon nitride sintered body which is produced by sintering a material in which at least an oxide of the Periodic Table Group 3a elements is incorporated in the silicon nitride powder as a sintering additive, at two stages, i.e., primary and secondary sintering stages under predetermined conditions so as to provide the silicon nitride sintered body having high-temperature strength higher than that of a conventional silicon nitride sintered body, and relates to a method of producing such a silicon nitride sintered body.
2. Description of the Related Art
In general, a silicon nitride sintered body is known to be excellent in the high-temperature strength and toughness. Therefore, various studies have been conducted for using such silicon nitride sintered bodies as parts of structural members, such as a turbine rotor of a gas turbine, a nozzle, a duct, a combustion chamber and the like, which are operated in severe environments at high temperatures and high pressures.
For example, since combustion efficiency of the gas turbine is improved with a rise of inlet temperature at an entrance of the turbine, various studies of silicon nitride sintered bodies have been conducted for aiming at further improvements of their high-temperature strengths because they have high-temperature strengths greater than heat-resistant alloys such as Inconel and the like which have heretofore been used as the structure members and due to their relatively high toughness in comparison with the other ceramic materials.
Thus, the silicon nitride sintered body is originally excellent in high-temperature resistance, however, there have been proposals for improving the high-temperature strength.
Since the silicon nitride is a substance having high covalent bond and therefore poor in sinterability obtained by sintering a single component material thereof, it has generally been required for its sintering to beforehand mix it with a sintering additive, such as aluminum oxide, yttrium oxide, magnesium oxide and cerium oxide to improve the sinterability. However, the high-temperature characteristic is deteriorated due to the added sintering additive. Therefore, in order to improve the high-temperature characteristics, trials to incorporate oxides as sintering additives having high eutectic temperatures with SiO.sub.2 existing on a surface of silicon nitride have been carried out. However, it offers a problem that as the eutectic temperature increases, the sinterability decreases.
Furthermore, if it has a simple shape, it is possible to help sintering by a method incorporating a small amount of sintering additive and using a hot press. But, it is not suitable for a complicated part shape. Furthermore, the sintering additive exists in an amorphous state in the grain boundary of a silicon nitride sintered body, causing deterioration of the high-temperature strength. Therefore, various trials have been conducted for improving the high-temperature strength by crystallizing the sintering additive existing in the amorphous state, or by performing sintering with a sintering additive in an amount as small as possible.
Furthermore, such a study has been conducted as to improve the high-temperature strength of the silicon nitride sintered body by using a sintering additive excellent in high-temperature strength. For example, Japanese laid-open patent publication No. 61-201666 discloses that, in order to improve the high-temperature strength and toughness, oxide of hafnium and zirconium are dispersed in the matrix of silicon nitride in a specified ratio. Also, Japanese laid-open patent publication No. 62-153169 discloses that, in order to improve the high-temperature strength, oxides of rare-earth elements in a specified amount and at least one kind selected from the groups of oxides, carbides or suicides of Hf, Ta or Nb are added to a mixture of silicon nitride in a specified ratio.
However, sufficient high-temperature strength has not always been obtained only by using these sintering additives having high high-temperature strength.
Therefore, in Japanese Patent publication No. 5-15667, there is disclosed that, in order to improve the strength more than 20kg/mm.sup.2 compared to the conventional silicon nitride sintered body, by setting an average grain size and the maximum grain size of the silicon nitride powder and the sintering additive in a predetermined range. The proposal disclosed therein, however, aims mainly at the improvement of the room-temperature strength, and it cannot be the that it is sufficient with regard to the high-temperature strength.
Furthermore, studying the deterioration mechanism in association with the high-temperature characteristics of the silicon nitride sintered bodies produced by means of the conventional art from the view point of a systematic structure, FIG. 6 is the general structural view of the silicon nitride sintered body by means of the conventional art. As is obvious from FIG. 6(a), when a shear stress acts on silicon nitride particles in this state at a high temperature, grain boundary slip tends to occur as shown in FIG. 6(b) since only amorphous sintering additive exists in the gap between particles. Therefore, it may be considered that even if the silicon nitride sintered body is excellent in heat resistance compared to metals, it is poor in toughness.
On the other hand, as the conventional methods of manufacturing silicon nitride sintered bodies, there are disclosed in Japanese laid-open patent publication Nos. 63-100067, 63-206358, 6-234571, 6-287066 and 6-305838.
Japanese laid-open patent publication No. 63-100067 discloses that, in order to crystallize the silicon nitride grain boundary phase, a starting powder which is obtained by adding rare-earth oxides such as Y.sub.2 O.sub.3 to silicon nitride is fired at 1900.degree. C. for 2 hours in a N.sub.2 atmosphere of 10 atm., and then treated at 1400.degree. C. for 6 hours.
However, this method requires such an additional crystallizing treatment as to hold the grain boundary phase after lowering the temperature, since the sintering temperature is high, in order to crystallize the grain boundary phase.
Japanese laid-open publication No. 63-206358 discloses that at the time of hot-pressing the silicon nitride powder, the amount to be pressurized by the press is increased between a contraction-starting temperature of a silicon nitride powder molded article and a contraction-completed temperature thereof higher than the former.
In this method, however, since the sintering temperature is set to be increased at the latter half stage, abnormal grain growth tends to be caused.
Japanese laid-open patent publication No. 6-234571 discloses a crystallization procedure of the grain boundary phase by a sintering process at a temperature ranging from 1500 to 1950.degree. C. by a glass seal HIP technique, followed by a heat treatment in a non-oxidizing atmosphere at a temperature ranging from 1000 to 1600.degree. C.
In this method, however, first of all, the glass seal HIP method per se is complicated in its process, and the ambient pressure is not taken into account at all, therefore pores are easily caused.
Japanese laid-open patent publication No. 6-287066 discloses that the grain boundary phase of a silicon nitride crystal is crystallized by setting a temperature to fire the starting material containing silicon nitride, oxides of Group 3a elements and excessive oxygen higher than the eutectic temperature of the oxide of Group 3a elements and silicon nitride in a range from 1600 to 1950.degree. C., and setting the sintering ambient pressure not higher than 10 atm.
In this method, however, a sintering period of time is not taken into account, and it is impossible to prevent silicon nitride from being decomposed together with prevention of generation of pores only by making the ambient pressure not higher than 10 atm.
Japanese laid-open patent publication No. 6-305838 discloses that a molded article of mixture containing a silicon nitride is fired at a temperature range from 1500 to 1700.degree. C. in an atmosphere in which the whole pressure containing a nitrogen gas is from 0.1 to 2 atm till all open-pores disappear, followed by increase of the nitrogen atmospheric pressure and sintering temperature.
This method, however, is not suitable for a system not containing a sintering additive having a low melting point such as Al.sub.2 O.sub.3 and MgO, and tends to cause abnormal grain growth.
Furthermore, in either prior art which discloses two-stage sintering, there is no indication how to determine the temperature, time and ambient pressure to be made at the primary sintering stage.